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QUIC Working Group                                             P. Tiesel
Internet-Draft                                                 M. Palmer
Intended status: Informational                         B. Chandrasekaran
Expires: May 3, 2018                                         A. Feldmann
                                                               TU Berlin
                                                                  J. Ott
                                                               TU Munich
                                                        October 30, 2017


             Considerations for Unreliable Streams in QUIC
                draft-tiesel-quic-unreliable-streams-01

Abstract

   This memo outlines how to support unreliable streams as well as
   partially-reliable streams within QUIC.  The intention of this
   document is to collect requirements and considerations, to frame the
   design space, and to give an example how unreliable stream support
   could be realized.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on May 3, 2018.

Copyright Notice

   Copyright (c) 2017 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect



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   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Conventions and Definitions . . . . . . . . . . . . . . . . .   2
   2.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   3.  Modes of Unreliable Transmission  . . . . . . . . . . . . . .   3
   4.  Protocol Considerations . . . . . . . . . . . . . . . . . . .   3
     4.1.  Unreliable Stream Support Negotiation . . . . . . . . . .   4
     4.2.  Stream as a Message . . . . . . . . . . . . . . . . . . .   4
     4.3.  Stream ID 0x0 . . . . . . . . . . . . . . . . . . . . . .   4
     4.4.  Congestion Control on Unreliable Streams  . . . . . . . .   4
     4.5.  Flow Control on Unreliable Streams  . . . . . . . . . . .   4
     4.6.  Stream Open . . . . . . . . . . . . . . . . . . . . . . .   5
     4.7.  Retransmission of Partially-Reliable Stream Data  . . . .   5
     4.8.  Stream Close  . . . . . . . . . . . . . . . . . . . . . .   5
   5.  Application Interface Considerations  . . . . . . . . . . . .   5
     5.1.  Retransmissions within Unreliable Streams . . . . . . . .   5
     5.2.  Presentation of Unreliable Streams  . . . . . . . . . . .   6
     5.3.  Prioritization of Unreliable Streams  . . . . . . . . . .   6
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   7
   9.  Informative References  . . . . . . . . . . . . . . . . . . .   7
   Appendix A.  Implemenation Proposal . . . . . . . . . . . . . . .   8
     A.1.  Stream Identifiers  . . . . . . . . . . . . . . . . . . .   8
     A.2.  Stream Close  . . . . . . . . . . . . . . . . . . . . . .   9
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   9

1.  Conventions and Definitions

   The words "MUST", "MUST NOT", "SHALL", "SHALL NOT", "SHOULD", and
   "MAY" are used in this document.  It's not shouting; when these words
   are capitalized, they have a special meaning as defined in [RFC2119].

2.  Introduction

   This memo describes how QUIC can provide reliable, partially-
   reliable, and unreliable transmissions within the same connection.
   There are many use cases for unreliable delivery of stream data,
   e.g., to meet deadlines for data delivery in the presence of short-
   time congestion by avoiding head-of-line blocking.  For partial
   reliable streams, the sender can decide which frames to retransmit.
   This model allows applications to request the required kind of
   reliability on a per-stream level and to mix of reliable and



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   unreliable transmissions within the same stream.  Still, some control
   data or metadata often needs to be transmitted reliably.

   This draft is based on [I-D.draft-ietf-quic-transport-07] with the
   addition of uni-directional streams (https://github.com/quicwg/base-
   drafts/pull/885) and variable-length integer encoding
   (https://github.com/quicwg/base-drafts/pull/877).  It gives a
   comprehensive overview about considerations for adding support for
   unreliable streams to QUIC in a way that each stream is either fully
   reliable or unreliable (including partially-reliable).  Our
   implementation proposal in Appendix A needs minimal changes to the
   wire format - the third least significant bit of the Stream ID is
   repurposed to signal whether a stream is reliable or partial reliable
   / unreliable.

3.  Modes of Unreliable Transmission

   o  Reliable transmission suggests that all application data is re-
      transmitted if lost.

   o  Unreliable transmission suggests that no application data is re-
      transmitted if lost.

   o  Partial reliable transmission suggests that some application data
      is re-transmitted if lost.

   In principle, the way [I-D.draft-ietf-quic-recovery-06] realizes
   reliable transmission allows to realize all three levels of
   reliability mentioned above without changing the wire format.  The
   combination of packet-based acknowledgments with stream-based
   retransmits allows deciding on a per stream-frame or byte range base
   whether or how a lost stream frame is retransmitted.  For this memo,
   we define unreliable streams as streams, for which some or all lost
   stream frames are not re-transmitted.  Thus, our definition covers
   fully unreliable as well as partially-reliable transmission.

4.  Protocol Considerations

   For clear application semantic, the receiver must be able to know
   whether to rely on the sender to retransmit lost stream data.
   Therefore, an endpoint opening an unreliable stream MUST indicate
   that lost stream data might not be retransmitted.  The indication
   must either be carried on each frame, that could be received first by
   an endpoint, or the indication must be transmitted reliably and
   acknowledged before the first frame of an unreliable stream is sent.

   We anticipate two options to indicate whether a stream is unreliable:




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   o  Indicate unreliable streams within the Stream ID.

   o  Leave the signaling of unreliable streams completely to the
      application layer.

   The authors advocate for the former option.  This approach does not
   introduce complex interwinding between QUIC and the application.

4.1.  Unreliable Stream Support Negotiation

   Support of unreliable streams should be optional to reduce the
   complexity of a minimal QUIC implementation.  An endpoint signals its
   willingness to receiving unreliable stream frames during the TLS
   handshake using the transport parameter allow_unreliable_streams in
   analogy to the omit_connection_id flag specified in
   [I-D.draft-ietf-quic-transport-07].  An application that makes no use
   of partial delivery of stream data, should not signal willingness to
   allow unreliable streams.

4.2.  Stream as a Message

   Unreliable streams are particularly useful if QUIC streams are used
   as a message abstraction.  If the messages can be sent using a single
   stream frame on a unidirectional stream, the state committed at the
   sender-side and receiver-side can be minimized, and signaling of
   stream close can be omitted.  The loss of such a frame does not
   introduce state at the perceived receiver, nor does it require the
   sender to keep state for retransmission.

4.3.  Stream ID 0x0

   Data of stream 0x0 MUST be transmitted reliably as TLS expects
   reliable transmission.

4.4.  Congestion Control on Unreliable Streams

   Unreliable streams are subject to regular congestion control.

   It should be clarified that ACK and RST_STREAM Frames are not subject
   to congestion control.

4.5.  Flow Control on Unreliable Streams

   Unreliable streams are subject to regular flow control on connection
   and stream level.

   Note:  It is up to further discussion under which assumptions this
      can be relaxed.



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4.6.  Stream Open

   A receiver may want to treat reliable and unreliable streams
   differently, e.g., in the way state is represented or how the stream
   data is presented to the application.  Therefore, the receiver needs
   to know upon the reception of the first stream frame whether the
   respective stream is reliable or unreliable.

   As the first frame sent may be lost or re-ordered, it is not
   sufficient to mark the first stream frame.  Instead, it is necessary
   to either reliably transmit the fact whether a stream is reliable or
   unreliable or to have all frames of the stream annotated.

4.7.  Retransmission of Partially-Reliable Stream Data

   Retransmissions of partially-reliable stream data SHOULD always
   contain the same data, as was sent in the original transmission.

   Note:  It is up to further discussion under which assumptions this
      can be relaxed.

4.8.  Stream Close

   As frames of unreliable streams may not be retransmitted, the loss of
   a frame indicating the end of a stream may result in a "zombie"
   stream state.  A QUIC version with unreliable stream support MUST
   ensure that either such a zombie state does not occur or include a
   mechanism to clean up streams in such a zombie state, e.g., by using
   a window of active unreliable stream ids.

   The authors advocate for solving this issue by reliably transmitting
   the final offset of all streams, that consist of more than a single
   stream frame.  The retransmit of the stream end can be achieved by
   sending an empty stream frame with the final offset and the FIN bit
   set or by using an RST_STREAM frame.  For unidirectional streams that
   only consist of a single stream frame, retransmission of the final
   offset is not necessary - See Section 4.2 for details.

5.  Application Interface Considerations

5.1.  Retransmissions within Unreliable Streams

   While unreliable streams suggest just disabling retransmissions for
   these streams, applications may choose to apply arbitrary
   retransmission strategies for unreliable streams, e.g., retransmit
   stream data as long it will likely be delivered on-time with respect
   to an application provided deadline, or only retransmit certain byte
   ranges.



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   A QUIC implementation that implements retransmissions on a per-packet
   basis, therefore, may retransmit unreliable stream data even if not
   requested by the application.

5.2.  Presentation of Unreliable Streams

   The presentation of unreliable streams is application specific.  The
   anticipated use cases include the following.

   o  Data being delivered annotated with its offset as it is received.

   o  Data being delivered after a deadline with an annotated list of
      holes.

   o  Data being delivered as concatenation of the stream fragments
      received.  This can be useful if the application's framing is
      aligned with the QUIC stream frames.

5.3.  Prioritization of Unreliable Streams

   Prioritization of unreliable streams uses the same priority mechanism
   as reliable streams.

   Applications that want UDP-like behavior, and thus want to avoid data
   sent over unreliable streams queued in send buffers, must ensure
   that:

   o  The flow control windows are large enough to send at any given
      point in time

   o  The data rate sent in all frames stays below the one permitted by
      the congestion window.

   o  The priority of unreliable streams is high enough to transmit
      data, even if there are retransmissions outstanding on other
      streams.

   To enable writing portable applications, guidelines how
   prioritization should be handled in a QUIC implementation and how it
   is exposed to the application are required.

6.  Security Considerations

   Unreliable Streams open a few additional risks of information
   disclosure.






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   o  An active, on path attacker can drop selected frames and see
      whether the transmission timings change to see whether unreliable
      streams are used.

   o  Using streams as a message as described in Section 4.2 will most
      likely result in packets which sizes resemble the size of the
      individual message contained.  If not mitigated, this can disclose
      the individual message length.

7.  IANA Considerations

   TBD

8.  Acknowledgements

   This work has been supported in part by Leibniz Prize project funds
   of DFG - German Research Foundation: Gottfried Wilhelm Leibniz-Preis
   2011 (FKZ FE 570/4-1) and received funding from the European Union's
   Horizon 2020 research and innovation programme 2014-2018 under grant
   agreement No. 644866, Scalable and Secure Infrastructures for Cloud
   Operations (SSICLOPS).

9.  Informative References

   [I-D.draft-ietf-quic-applicability-01]
              Kuehlewind, M. and B. Trammell, "Applicability of the QUIC
              Transport Protocol", draft-ietf-quic-applicability-01
              (work in progress), October 2017.

   [I-D.draft-ietf-quic-recovery-06]
              Iyengar, J. and I. Swett, "QUIC Loss Detection and
              Congestion Control", draft-ietf-quic-recovery-06 (work in
              progress), September 2017.

   [I-D.draft-ietf-quic-transport-07]
              Iyengar, J. and M. Thomson, "QUIC: A UDP-Based Multiplexed
              and Secure Transport", draft-ietf-quic-transport-07 (work
              in progress), October 2017.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.








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Appendix A.  Implemenation Proposal

   This proposal realizes unreliable streams by modifying
   [I-D.draft-ietf-quic-transport-07] with the addition of uni-
   directional streams (https://github.com/quicwg/base-drafts/pull/872)
   and variable-length integer encoding (https://github.com/quicwg/base-
   drafts/pull/877) to support unreliable streams.  It modifies the
   Stream ID to signal whether a stream is reliable in analogy to the
   way unidirectional streams are signaled.  In addition, it addresses
   the Stream Close concerns raised in Section 4.8.

A.1.  Stream Identifiers

   Streams are identified by a variable-length integer, referred to as
   the Stream ID.  The least significant three bits of the Stream ID are
   used to identify the type of stream (unidirectional or
   bidirectional), (unreliable or reliable) and the initiator of the
   stream.

   The second least significant bit (0x2) of the Stream ID
   differentiates between unidirectional streams and bidirectional
   streams.  Unidirectional streams always have this bit set to 1 and
   bidirectional streams have this bit set to 0.

   The third least significant bit (0x4) of the Stream ID differentiates
   between unreliable and reliable streams.  Unreliable streams always
   have this bit set to 1 and reliable streams have this bit set to 0.

   The three type bits from a Stream ID, therefore, identify streams as
   summarized in Appendix A.2.





















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        +----------+----------------------------------------------+
        | Low Bits | Stream Type                                  |
        +----------+----------------------------------------------+
        | 0x0      | Client-Initiated, Bidirectional , Reliable   |
        |          |                                              |
        | 0x1      | Server-Initiated, Bidirectional , Reliable   |
        |          |                                              |
        | 0x2      | Client-Initiated, Unidirectional, Reliable   |
        |          |                                              |
        | 0x3      | Server-Initiated, Unidirectional, Reliable   |
        |          |                                              |
        | 0x4      | Client-Initiated, Bidirectional , Unreliable |
        |          |                                              |
        | 0x5      | Server-Initiated, Bidirectional , Unreliable |
        |          |                                              |
        | 0x6      | Client-Initiated, Unidirectional, Unreliable |
        |          |                                              |
        | 0x7      | Server-Initiated, Unidirectional, Unreliable |
        +----------+----------------------------------------------+

A.2.  Stream Close

   Streams MUST be explicitly closed.  This can either be done by
   setting the FIN bit on the frame carrying the final offset of the
   stream or by using an RST_STREAM frame indicating the final offset.

   For unreliable streams transmitted using more than one stream frame,
   the stream close has to be transmitted reliably to prevent zombie
   stream state.  If the packet containing the FIN flagged stream frame
   is lost, and the data in this stream is not to be retransmitted, the
   sender can (re-)transmit the stream close by sending an empty FIN
   flagged stream frame carrying the final offset.

Authors' Addresses

   Philipp S. Tiesel
   TU Berlin
   Marchstr. 23
   Berlin
   Germany

   Email: philipp@inet.tu-berlin.de









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   Mirko Palmer
   TU Berlin
   Marchstr. 23
   Berlin
   Germany

   Email: mirko@inet.tu-berlin.de


   Balakrishnan Chandrasekaran
   TU Berlin
   Marchstr. 23
   Berlin
   Germany

   Email: balac@inet.tu-berlin.de


   Anja Feldmann
   TU Berlin
   Marchstr. 23
   Berlin
   Germany

   Email: anja@inet.tu-berlin.de


   Joerg Ott
   TU Munich
   Boltzmannstrasse 3
   Garching bei Muenchen
   Germany

   Email: ott@in.tum.de

















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